Remove dependency files attribute from generated report

What does this MR do?

Dependency files will be removed as of security report schemas version 15-1-0. This MR removes the attribute from the generated report and updates the structs to remove no longer used attributes.

This version will be used in gemnasium to generate 15-1-0 compatible dependency scanning reports.

What are the relevant issue numbers?

• Remove the `dependency_files` property from the... (gitlab-org/gitlab#439770 - closed) • Olivier Gonzalez • 17.0 • On track
• Update Gemnasium analyzer to adopt the new secu... (gitlab-org/gitlab#439777 - closed) • Igor Frenkel • 17.0

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer